Assessment of concrete bridges -Structural capacity.
Experiences from full-scale testing to failure of a bridge in Kiruna
Niklas Bagge1 MSc, PhD Student Jonny Nilimaa1 PhD, Ass. Sr. Lect. Gabriel Sas1 PhD, Ass. Sr. Lect. Thomas Blanksvärd1 PhD, Assoc. Professor Lars Bernspång1 PhD, Assoc. Professor Björn Täljsten1
PhD, Professor Anders Carolin 2
PhD, Bridge Engineer,
Lennart Elfgren1 PhD, Em. Professor 1 Luleå University of Technology, SE-971 87 Luleå, name.name@ltu.se
2 Trafikverket, SE-972 42 Luleå, anders.carolin@trafikverket.se ABSTRACT
To calibrate methods for condition assessment of prestressed concrete (PC) bridges, tests were carried out on a 55 year old five-span bridge with a length of 121 m in Kiruna in northern Sweden. Both non-destructive and destructive full-scale tests were performed. This paper presents results regarding methods for assessment of the structural capacity of concrete bridges. Key words: Assessment, Cracking, Corrosion, Creep, Modelling, Pre-stress, Reinforcement, Structural Design, Sustainability, Testing.
1. INTRODUCTION 1.1 General
Assessment, repair and strengthening of existing bridges are required in order to meet current and future demands on sustainability of existing infrastructure. For instance, a survey carried out within the project MAINLINE (2015) [1], indicated a need in Europe for strengthening of 1500 bridges, replacement of 4500 bridges and replacement of 3000 bridge decks. It is believed that for some of these structures replacement can be avoided if more accurate assessment methods are used, see e.g. Sustainable Bridges (2007) [2], Nilimaa et al. (2016) [3] and Paulsson et al.(2016) [4].
Figure 1. The Kiruna Bridge. Left: Photo with road E10 in the foreground behind birches, the earlier location of the Iron Ore Railway line and the LKAB iron mine in the background. Right: Load-deflection diagrams for the tested span (No 2 from the right): dashed thin blue - initial Non Linear Finite Element Analysis (NLFEA); black circles – updated NLFEA; black triangles with green line – updated NLFEA with refined mesh; and dashed thick red line – test results. Bagge (2017) [5].
1.2 The Kiruna Bridge
The test object, located in Kiruna, Sweden, was a viaduct across the European road E10, see Figure 1. It was constructed in 1959 as a part of the road connecting the city centre and the LKAB mining area. Due to underground mining, the entire area underwent excessive settlement. The owner of the bridge, LKAB, closed it in October 2013 and planned to tear it down in September 2014. Before demolition, the bridge was tested in a research project which will be discussed below [5]-[7].
2 ASSESSMENT METHODS
Structural analysis has a crucial role in bridge assessments. To support rational improvements of this analysis, a multi-level strategy has been developed, Bagge (2017) [5]. By gradually increasing the complexity, the structural response and the load-carrying capacity can be more accurately estimated. This methodology, see Figure 2, is an extension of the strategy developed for bridges in SB-LRA (2007) [8], MAINLINE 2014 [1], UIC 2009 [10], ISO 2015 [11], fib Model Code 2010 (2013) [12] and a procedure specialised for deck slabs by Plos et al. (2016) [9]. Various degrees of degradation and/or fatigue must also be taken into consideration, see e.g. Elfgren (2015) [14].
In order to enable a better representation of the structural behaviour in the intermediate Phase 2 of the assessment, three sublevels have been defined by Bagge (2017) [6]:
2A - Linear elastic analysis
2B - Linear elastic analysis with limited redistribution 2C - Plastic analysis
These methods utilize well-established procedures. For instance, methods for linear elastic analysis with limited redistribution are provided by design codes as e.g. the fib Model Code 2010 [12] and for plastic analysis by K.W.Johansen and M.P. Nielsen [13].
Figure 2. Assessment procedure in three phases: (1) Initial, (2) Intermediate and (3) Enhanced. Based on Sustainable Bridges 2007 [2], [8], Mainline 2014 [1], UIC 2009 [10], ISO 2015 [11], and Paulsson et al (2016) [4].
For structural assessment at enhanced levels, Phase 3, the FE method is a powerful tool. However, depending on the idealisations of the structure, different types of failures can be captured.. Since different modelling choices can necessitate varying computational demands, it is useful to define three levels of approximation for the enhanced structural analysis in Phase 3: 3A - Nonlinear finite elements analysis NLFEA able to calculate the capacity related to flexure. 3B - As 3A but also able to calculate shear-related failures
3C - As 3B but also able to calculate anchorage related failures.
In Figure 1 (right) four load-deflection diagrams are shown for the tested span 2 in the bridge. A Phase 3A initial Non Linear Finite Element Analysis (NLFEA) is shown with a thin dashed blue line. The model was updated with improved material and boundary conditions (partially built in columns) in Phase 3B, shown with black circles. Here a shear failure was captured which did not show up in Phase 3A. Smaller elements were also tested shown with black triangles and a green line. Finally the obtained test result is shown with a thick dashed red line. The Phase 3B with elements of ordinary size (black circles) showed the best correspondence to the test results. 4 SUMMARY AND CONCLUSIONS
A stepwise procedure is recommended for assessment with non-linear finite element calculations. Further research on assessment methods and life-cycle analysis of bridges should be carried out in order to get a more sustainable management.
ACKNOWLEDGEMENT
The authors gratefully acknowledge financial support from Trafikverket/BBT, LKAB/HLRC, SBUF and LTU. They also thank colleagues in the Swedish Universities of the Built Environment (Chalmers University of Technology in Göteborg, the Royal Institute of Technology (KTH) in Stockholm and Lund Institute of Technology (LTH) in Lund) for fruitful cooperation. The experimental work and a previous monitoring campaign were carried out in cooperation with staff of Complab at Luleå University of Technology.
REFERENCES
[1] MAINLINE. A European FP 7 research project 2011-2014 with 19 partners with the full name: MAINtenance, renewaL and Improvement of rail transport iNfrastructure to reduce Economic and environmental impacts. 27 deliverables. www.mainline-project.eu
[2] Sustainable Bridges – Assessment for Future Traffic Demands and Longer Lives. A European FP 6 Research Project during 2003-2007 with 32 partners. Four guidelines and some 35 background documents are available at www.sustainablebridges.net.
[3] Nilimaa, J., Bagge, N., Häggström, J., Blanksvärd, Th., Sas, Gabriel., Täljsten, B., Elfgren, L. (2016). ”More realistic Codes for Exisiting Bridges”. 19th IABSE Congress, (IABSE), Zürich. Ed. by L. Elfgren, J. Jonsson, M. Karlsson, L. Rydberg-Forssbeck & B. Sigfid, pp 399-407, http://ltu.diva-portal.org/smash/get/diva2:1073956/FULLTEXT01.pdf
[4] Paulsson, B., Bell, B., Schewe, B., Jensen, J. S., Carolin, A. & Elfgren, L. (2016). “Results and Experiences from European Research on Railway Bridges” 19th IABSE Congress, Stockholm, 21-23 September 2016. Zürich, pp. 2570 – 2578. ISBN 978-3-85748-144-4.
http://www.diva-portal.org/smash/get/diva2:1015045/FULLTEXT02.pdf
[5] Bagge Niklas (2017). Structural Assessment Procedures for Existing Concrete Bridges.
Implementation of experiences from tests to failure of the Kiruna Bridge. PhD thesis.
Luleå: Luleå University of Technology; to be published.
[6] Bagge, N., Nilimaa, J., Blanksvärd, T., & Elfgren, L. (2014). “Instrumentation and full-scale test of a post-tensioned concrete bridge”. Nordic Concrete Research, 51, 63-83. [7] Nilimaa Jonny (2015) Concrete Bridges: Improved Load Capacity. Doctoral Thesis.
Luleå: Luleå University of Technology; 2015, 176 p. http://www.diva-portal.org/smash
[8] SB-LRA (2007). "Guideline for load and resistance assessment of existing European railway bridges." Sustainable Bridges, 428 pp. www.sustainablebridges.net.
[9] Plos, M., Shu, J., Zandi, K. Z. & Lundgren, K. (2016). A multi-level structural assessment strategy for reinforced concrete bridge deck slabs. Structure and Infrastructure
Engineering, pp 1-19.
[10] UIC (2009). "Defects in railway bridges and procedures for maintenance." UIC Code
778-4 R. International Union of Railways (UIC), Paris, France, 32 pp.
[11] ISO (2015). "General principles on reliability of structures." ISO 2394:2015, International Organization for Standardization (ISO), Switzerland.
[12] fib Model Code 2010 (2013). fib Model Code for concrete structures 2010, Ernst & Sohn, Berlin, Germany. ISBN 978-3-433-03061-5, 402 pp.
[13] Nielsen, M.P. & Hoang, L.C. (2016). Limit Analysis and Concrete Plasticity. 3rd Ed. CRC Press, London, United Kingdom, 816 pp.
[14] Elfgren, Lennart (2015). Fatigue Capacity of Concrete Structures: Assessment of Railway Bridges. Research Report, Luleå University of Technology, 103 pp. http://www.diva-portal.se/smash/get/diva2:997704/FULLTEXT02.pdf
